The present invention relates to an encapsulant comprising an epoxy resin composition with good moldability and high reliability in use, and semiconductor devices in which such an encapsulant is applied.
While there is a steady trend toward reduction of size and thickness of semiconductor elements, we can also note a tendency for the number of pins to increase in the elements in line with the enhanced performance of these elements. This has mandatorily narrowed the interval of the outer leads connecting the semiconductor elements to the substrate, making substrate packaging harder to work. As a solution to this problem, it has been proposed to employ the outer shape designated "BGA" (ball grid array package) in place of conventional "SOP" (small outline package) or "QFP" (quad flat package), and certain elements of this outer shape are already in quantity production in some quarters.
BGA allows wide spacing of the soldering balls for joint as these soldering balls are formed at a part or over the whole surface of the backside of an element, so that this outer shape is suited for multi-pin packaging. However, since BGA is designed to apply an encapsulant on one side of a BT (bismaleimide triazine) substrate or a glass-reinforced epoxy substrate, the produced package necessarily has a bimetal structure and is therefore subject to warping due to the difference in coefficient of contraction between the substrate and the encapsulant.
In order to overcome this problem, various proposals have been made, such as combining a polyfunctional epoxy compound with a specific curing agent and/or a curing accelerator (JP-A-8-176277), or mixing the encapsulants differing by more than 20.degree. C. in glass transition temperature (JP-A-8-213518), but none of these proposals have been found satisfactory.